If Lincoln did not have Johnson as his Vice Presidential candidate, which was a calculated bet, as the Southern Governor of Tennessee and the sole southern supporter of the Union, he would never have won the election of 1864, or at least so it goes.
On the other hand, by incorporating Johnson into it, when Lincoln was ultimately assassinated, some five months after the election, Johnson became the President and proceeded to punish the Southern states, extend the massive reprisals, and defy Congress. Had it been Hannibal Hamlin, the Vice President for Lincoln’s first term, the post war reconstruction would have been driven by an abolitionist who firmly believed, as did Lincoln, that slavery was intolerable. Instead, we ended up with one hundred years of civil rights warfare precipitated by Johnson and a Johnson’s impeachment.
Of course, had Lincoln not gotten elected, not only might the nation not have survived as a unified nation, with the south and slavery intact, but we would not have had the Thirteenth, Fourteenth, or Fifteenth Amendments.
History appears to play itself out in less than clear ways